Half of Earth’s Heat from Radioactive Decay

Nearly half of the Earth’s heat comes from the radioactive decay of materials inside. Image Credit: Lawrence Berkeley National Laboratory

Nearly half of the Earth’s heat comes from the radioactive decay of materials inside, according to a large international research collaboration that includes a Kansas State University physicist. Studying the physical properties of Earth can help astrobiologists understand the mechanisms that caused our planet to become habitable. In turn, this information can then be used to determine where and how to search for habitable worlds throughout the Universe.

Glenn Horton-Smith, associate professor of physics, was part of a team gathering some of the most precise measurements of the Earth’s radioactivity to date by observing the activity of subatomic particles — particularly uranium, thorium and potassium. Their work appears in the July issue of Nature Geoscience in the article "Partial radiogenic heat model for Earth revealed by geoneutrino measurements."

"It is a high enough precision measurement that we can make a good estimate of the total amount of heat being produced by these fissions going on in naturally occurring uranium and thorium," Horton-Smith said.

Itaru Shimizu of Tohoku University in Sendai, Japan, and collaborating physicists, including Horton-Smith, made the measurement using the KamLAND neutrino detector in Japan. KamLAND, short for Kamioka Liquid-Scintillator Antineutrino Detector, is an experiment at the Kamioka Observatory, an underground neutrino observatory in Toyama, Japan. Neutrinos are neutral elementary particles that come from nuclear reactions or radioactive decay. Because of their small size, large detectors are needed to capture and measure them.

Horton-Smith was involved with developing the KamLAND detector from 1998 to 2000 and he helped prepare it to begin taking data in 2002. Several years later, he was involved in an upgrade of the detector to help it detect solar neutrinos. For the most recent project, Horton-Smith’s role was to help keep the detector running and taking measurements from nuclear reactors in Japan.

A drawing of the KamLAND detector components. The balloon is filled with 1000tons of liquid scintillator and is surrounded by 1879 PMTs mounted on a steel sphere. The liquid scintillator acts as both the target and detection volume. The volume between the balloon and the steel sphere is filled with non-scintillating mineral oil – this acts as a shield from low radioactivity coming from the surrounding rocks and the PMTs themselves. Finally, the volume between the steel sphere and the rock has a third layer, it’s filled with water with PMTs mounted on the cylindrical surface on the outside KamLAND. This final layer uses Cherenkov radiation to detect muons passing through the detector. The muons can interact with the material in the central detector producing background radiation. By knowing exactly when a muon passes through KamLAND, the detector volume can be vetoed, to avoid detecting the background. Credit: Lawrence Berkeley Laboratories/ http://kamland.lbl.gov

By gathering measurements of radioactive decay, the KamLAND researchers were able to observe geoneutrinos, or neutrinos from a geological source. They gathered data from 2002 to 2009 and had published their preliminary findings in Nature in 2005.

"That was the first time that observation of excess antineutrinos and a neutrino experiment were attributed to geoneutrinos," Horton-Smith said.

Previous research has shown that Earth’s total heat output is about 44 terawatts, or 44 trillion watts. The KamLAND researchers found roughly half of that — 29 terawatts — comes from radioactive decay of uranium, thorium and other materials, meaning that about 50 percent of the earth’s heat comes from geoneutrinos.

The researchers estimate that the other half of the earth’s heat comes from primordial sources left over when the earth formed and from other sources of heat. Earth’s heat is the cause behind plate movement, magnetic fields, volcanoes and seafloor spreading.

"These results helps geologists understand a model for the earth’s interior," Horton-Smith said. "Understanding the earth’s heat source and where it is being produced affects models for the earth’s magnetic field, too."

The research also provides better insight for instances when materials within the earth undergo natural nuclear reactions. Based on their research, the physicists placed a five-terawatt limit on the heat cause by such reactions, meaning that if there is any geological heating from nuclear reactors in the Earth’s core it is quite small when compared to heat from ordinary radioactive decay.

Horton-Smith is also involved with the Institute for the Physics and Mathematics of the Universe at Tokyo University in Kashiwa, Japan. He is leading a K-State exploration on the Double Chooz neutrino detector, which measure neutrinos from the Chooz nuclear power plant in the Ardennes region of northern France.